For many years it has been known to make cells with lithium metal anodes, and cathodes of a material into which lithium ions can be intercalated or inserted. A wide variety of intercalation materials are known as cathode materials, such as lithium cobalt oxide, and lithium manganese oxide, and such materials may be mixed with solid electrolyte material to form a composite cathode. Such cells may use a separator such as filter paper or polypropylene saturated with, as electrolyte, a solution of a lithium salt in an organic liquid such as propylene carbonate. Alternatively a polymer-based solid electrolyte may be used, for example a complex of a lithium salt with poly-(ethylene oxide). In the case of secondary or rechargeable lithium cells, the use of lithium metal anodes is unsatisfactory as problems arise from dendrite growth, but the use of an intercalation material in the anode, such as graphite, has enabled satisfactory cells to be made. Such cells may be referred to as “lithium ion” cells, or “swing” cells, as lithium ions are exchanged between the two intercalation materials during charge and discharge.
Polymer electrolytes based on polyvinylidene fluoride and related copolymers have also been proposed, for example by Gozdz et al. (U.S. Pat. No. 5,296,318), in which a copolymer of 75 to 92 percent vinylidene fluoride and 8 to 25 percent hexafluoropropylene, blended with a lithium salt and a compatible solvent such as ethylene carbonate/propylene carbonate mixture is cast from solution in a low boiling-point solvent such as tetrahydrofuran. GB 2 309 703 B (AEA Technology) describes a similar electrolyte composition in which the polymer is polyvinylidene fluoride (PVdF) homopolymer, the PVdF being characterized by a very low melt flow index. It is also possible to make such a solid polymer electrolyte by first making a porous film of the polymer material, and then immersing the film in a solution of lithium salt in an organic solvent so the electrolyte solution combines with the polymer film, as described in EP 0 730 316 A (Elf Atochem).